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Wet Moon
A wet moon (also called a Cheshire moon) is the visual phenomenon when the "horns" of the crescent Moon point up at an angle, away from the horizon, so that the crescent takes on the appearance of a bowl or smile. A wet moon occurs when the crescent Moon is low above the horizon and at a point more or less directly above the Sun's (invisible) position below the horizon. This in turn is determined by the positions of the Moon and Earth in their respective orbits, the inclinations of these orbits relative to one another and to Earth's celestial equator, and the observer's latitude on Earth. Wet moons occur routinely in the tropics (where the Sun and Moon rise and set nearly vertically), but rarely in the polar regions (where the Sun and Moon rise and set at a glancing angle or not at all). By contrast, a dry moon is one where the crescent of the moon is at any other angle. Name The terms ''wet moon'' and ''dry moon'' originate from Hawaiian mythology, where it was thought that ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Moon Paths
The Moon is Earth's only natural satellite. It orbits around Earth at an average distance of (; about 30 times Earth's diameter). The Moon rotates, with a rotation period (lunar day) that is synchronized to its orbital period (lunar month) of 29.5 Earth days. This is the product of Earth's gravitation having tidally pulled on the Moon until one part of it stopped rotating away from the near side, making always the same lunar surface face Earth. Conversley, the gravitational pull of the Moon, on Earth, is the main driver of Earth's tides. In geophysical terms, the Moon is a planetary-mass object or satellite planet. Its mass is 1.2% that of the Earth, and its diameter is , roughly one-quarter of Earth's (about as wide as the contiguous United States). Within the Solar System, it is the largest and most massive satellite in relation to its parent planet, the fifth-largest and fifth-most massive moon overall, and larger and more massive than all known dwarf planets. ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Libration
In lunar astronomy, libration is the cyclic variation in the apparent position of the Moon that is perceived by observers on the Earth and caused by changes between the orbital and rotational planes of the moon. It causes an observer to see slightly different hemispheres of the surface at different times. It is similar in both cause and effect to the changes in the Moon's apparent size because of changes in lunar distance (astronomy), distance. It is caused by three mechanisms detailed below, two of which cause a relatively tiny physical libration via tidal forces exerted by the Earth. Such true librations are known as well for other moons with locked rotation. The quite different phenomenon of a trojan asteroid's movement has been called ''Trojan libration'', and ''Trojan libration point'' means Lagrangian point. Lunar libration The Moon keeps one wikt:hemisphere, hemisphere of itself facing the Earth because of tidal locking. Therefore, the first view of the far side of ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Blue Moon
A blue moon refers either to the presence of a second full moon in a calendar month, to the third full moon in a season containing four, or to a moon that appears blue due to atmospheric effects. The calendrical meaning of "blue moon" is unconnected to the other meanings. It is often referred to as “traditional”, but since no occurrences are known prior to 1937 it is better described as an invented tradition or “modern American folklore”. The practice of designating the second full moon in a month as "blue" originated with amateur astronomer James Hugh Pruett in 1946. It does not come from Native American lunar tradition, as is sometimes supposed. The moon – not necessarily full – can sometimes appear blue due to atmospheric emissions from large forest fires or volcanoes, though the phenomenon is rare and unpredictable (hence the saying “once in a blue moon”). A calendrical blue moon (by Pruett's definition) is predictable and relatively common, happening 7 t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Black Moon
Black moon is a term first recorded in 2016. It is not a term used in astronomy. No single, universally accepted definition exists. Among the meanings ascribed to it are these: a second new moon that appears in the same month; the third new moon in an astronomical season with four new moons; the absence of a new moon in February; or the absence of a full moon in February. Definitions, frequency, dates Month with two new moons One use of the term is for the occurrence of a second new moon in a calendar month. This is analogous to the by-month definition of a blue moon as the second full moon in a month. February is too short for a second new moon to occur. This event occurs about every 29 months. The assignment of a calendar date to a new moon, and in which month a second new moon occurs, depends on the time zone. For example, the new moon of 2016-10-01T00:11 UTC occurs on 1 October for Europe, Asia and Oceania, making it the first of two new moons in October. However, fo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Equinox
A solar equinox is a moment in time when the Sun appears directly above the equator, rather than to its north or south. On the day of the equinox, the Sun appears to rise directly east and set directly west. This occurs twice each year, around 20 March and 23 September. An equinox is equivalently defined as the time when the plane of Earth's equator passes through the geometric center of the Sun's disk. This is also the moment when Earth's rotation axis is directly perpendicular to the Sun-Earth line, tilting neither toward nor away from the Sun. In modern times, since the Moon (and to a lesser extent the planets) causes Earth's orbit to vary slightly from a perfect ellipse, the equinox is officially defined by the Sun's more regular ecliptic longitude rather than by its declination. The instants of the equinoxes are currently defined to be when the apparent geocentric longitude of the Sun is 0° and 180°. The word is derived from the Latin ', from ' (equal) and ' ( ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Nodal Precession
Nodal precession is the precession of the orbital plane of a satellite around the rotational axis of an astronomical body such as Earth. This precession is due to the non-spherical nature of a rotating body, which creates a non-uniform gravitational field. The following discussion relates to low Earth orbit of artificial satellites, which have no measurable effect on the motion of Earth. The nodal precession of more massive, natural satellites like the Moon is more complex. Around a spherical body, an orbital plane would remain fixed in space around the gravitational primary body. However, most bodies rotate, which causes an equatorial bulge. This bulge creates a gravitational effect that causes orbits to precess around the rotational axis of the primary body. The direction of precession is opposite the direction of revolution. For a typical prograde orbit around Earth (that is, in the direction of primary body's rotation), the longitude of the ascending node decreases, that ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Lunar Node
A lunar node is either of the two orbital nodes of the Moon; that is, the two points at which the orbit of the Moon intersects the ecliptic. The ''ascending'' (or ''north'') node is where the Moon moves into the northern ecliptic hemisphere, while the ''descending'' (or ''south'') node is where the Moon enters the southern ecliptic hemisphere. Motion The line of nodes, which is also the intersection between the two respective planes, rotates (precesses) with a period of 18.6 years or 19.35° per year. When viewed from the celestial north, the nodes move clockwise around Earth, I.e. with a retrograde motion (opposite to Earth's own spin and its revolution around the Sun). So the time from one node crossing to the next (see eclipse season) is approximately a half-year minus half of 19.1 days -- or about 173 days. Because the orbital plane of the Moon precesses in space, the lunar nodes also precess around the ecliptic, completing one revolution (called a ''draconic period' ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Intersection (Euclidean Geometry)
In geometry, an intersection is a point, line, or curve common to two or more objects (such as lines, curves, planes, and surfaces). The simplest case in Euclidean geometry is the line–line intersection between two distinct lines, which either is one point (sometimes called a '' vertex'') or does not exist (if the lines are parallel). Other types of geometric intersection include: * Line–plane intersection * Line–sphere intersection * Intersection of a polyhedron with a line * Line segment intersection * Intersection curve Determination of the intersection of flats – linear geometric objects embedded in a higher-dimensional space – is a simple task of linear algebra, namely the solution of a system of linear equations. In general the determination of an intersection leads to non-linear equations, which can be solved numerically, for example using Newton iteration. Intersection problems between a line and a conic section (circle, ellipse, parabola, etc.) or ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Orbital Nodes
An orbital node is either of the two points where an orbit intersects a plane of reference to which it is inclined. A non-inclined orbit, which is contained in the reference plane, has no nodes. Planes of reference Common planes of reference include the following: * For a geocentric orbit, Earth's equatorial plane. In this case, non-inclined orbits are called ''equatorial''. * For a heliocentric orbit, the ecliptic or invariable plane. In this case, non-inclined orbits are called ''ecliptic''. * For an orbit outside the Solar System, the plane through the primary perpendicular to a line through the observer and the primary (called the ''plane of the sky''). Node distinction If a reference direction from one side of the plane of reference to the other is defined, the two nodes can be distinguished. For geocentric and heliocentric orbits, the ascending node (or north node) is where the orbiting object moves north through the plane of reference, and the descending node (or so ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Sidereal Month
In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month. Variations In Shona, Middle Eastern, and European traditions, the month starts when the young crescent moon first becomes visible, at evening, after conjunction with the Sun one or two days before that evening (e.g., in the Islamic calendar). In ancient Egypt, the lunar month began on the day when the waning moon could no longer be seen just before sunrise. Others run from full moon to full moon. Yet others use calculation, of varying degrees of sophistication, for example, the Hebrew calendar, the Chinese calendar, or the ecclesiastical lunar calendar. Calendars count integer days, so months may be 29 or 30 days in length, in some regular or irregular sequence. Lunar cycles are prominent, and calculated with great precision in the ancient Hindu Panchangam calendar, widely use ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Orbit Of The Moon
The Moon orbits Earth in the retrograde and prograde motion, prograde direction and completes one orbital period, revolution relative to the March Equinox, Vernal Equinox and the fixed stars in about 27.3 days (a tropical month and sidereal month), and one revolution relative to the Sun in about 29.5 days (a synodic month). On average, the lunar distance, distance to the Moon is about from Earth's centre, which corresponds to about 60 Earth radii or 1.28 light-seconds. Earth and the Moon orbit about their barycentre (common centre of mass), which lies about from Earth's centre (about 73% of its radius), forming a satellite system called the Earth–Moon system. With a mean orbital speed around the barycentre of , the Moon covers a distance of approximately its diameter, or about half a degree on the celestial sphere, each hour. The Moon differs from most Regular moon, regular satellites of other planets in that its orbital plane (astronomy), orbital plane is closer to the ec ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
